Distortionless amplifier



Oct. 23, 1934, w. A. KNOOP DISTORTIONLESS AMPLIFIER Filed April 18, 1931Fla. /1

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INVENTOR W. A. KNOOP BY A TTORNE V Patented Oct. 23, 1934 UNITED STATESPATENT OFFICE Application April 18, 1931, Serial No. 531,106

5 Claims.

This invention relates to repeating devices arranged to be operated bydistortionless amplifiers of the space dischargetype.

An object of this invention is to compensate for distortion introducedin the signals by a vacuum tube amplifier, whereby the gridpotentialplate current characteristic is made to approximate a straightline.

, In an amplifying and repeating system it is desirable to have asnearly as possible a distortionless or straight-line amplifyingcharacteristic in order that the original signals be faithfullyretransmitted. The response of the sys tem toincoming signals ofpositive polarity should "be of the same order as the response toincoming signals of negative polarities. When substantial, departurefrom this condition exists, the amplified signals are, of course,distorted. An amplifying system having a'curved characteristic producesanamplified signal which is not symmetrical with respect to a normalno-signal output. In general, the positive impulses of an incomingsignal are amplified more than the negative impulses and the result ofsuch unsymmetrical amplification has been found to be a time-distortionof the component impulses of the incoming signal. For example, when arepeating relay is employed to retransmit such time-distorted signals,the retransmitted signal "differs materially in length of componentimpulses from theoriginal incoming signal. Timedistortion of this sortis not permissible in the art of faithful amplification and repeating ofsignals and has contributed to the needfor dis- "tortionless amplifyingsystems in communicapensator'tubearranged inparallel, andthe out: putvoltage or .the. compensator tube is superimf Heretofore various methodsof and means for compensating for the distortion introduced in signalsby vacuum tube amplifiers have been employed, such asthe known push-pullarrangement of vacuum tubes, and the compensating tube arrangementdisclosed in U. syPatent 1,856,373, May 3, 1932, toE. TgBurton. In thepush-pull arrangement the distortion introduced in the positive signalsby one tube is equalized by the. distortion introducedflin the negativesignals by the other tube. so that the ,output signals of positive andnegative polarities are symmetrical. In

- the compensating tube arrangements of the" above patentthe inputvoltage is impressed on.

the grid circuits of .an amplifier and a composed on the signal intheinput orthe output circuit or the amplifying tube 'to make the inputvoltage-output current characteristic of the system symmetrical on bothsides of the zero point of the operating range of the input voltage.

According to the present invention two similar vacuum tubes are arrangedto have their output circuits interconnected in opposite relationthrough separate windings of an impulse repeating device in such amanner that the normal space currents of both vacuum tubes are bal-.anced. The two tubes have common sources of normal grid voltage, normalplate voltage and filament heating energy. Hence, when the normal spacecurrent effects are once balanced, this balance is substantiallymaintained when the normal voltage sources change, for example, whenplateor grid batteries deteriorate, or when the filament heating energymay vary, as the tubes have similar characteristics. In addition,,asuit; able resistance is connected in the common output connection ofthe two tubes, thereby providing a resistance coupling of said tubes.The effect ofthis resistance will be discussed in detail below. Theimpulses to be repeated are impressed on only one tube so that theamplifier tube, which is normally balanced by what may be termed abalancingtube, is in a condition to produce an amplifier output currentfree of any distortion which might be introduced by such amplifier tube,tooperate the repeating device. A better understanding of theinventionwill be had from thefollowing detailed description and appendedclaims when taken in conjunction with the accompanying drawing of which:

Fig. 1 showsa preferredembodiment of the invention; and I Fig. 2 showsseveral graphs of the grid potentialeplate current characteristics ofthe amplifier circuitshown in Fig. 1. Referring to Fig. 1', theamplifiertubell is arranged twithitsinput circuit connected to the conductors 19,which designate any system over' whichan incoming signal, whichitgisdesired to amplifyandfretransmit, is received. The plate circuit oftube 11 extends through the right hand windings of a three-positionpolar relay 12,'meter 13,1resistance element 14, battery 15, and back toits filament and plate. The balancing'tube 16 has its platecircuitarranged to extend through the left 'hand windings of relay 12,meter 13, resistance element 14, battery 15 andback to its filament,and, plate. The plate circuits are respectively connected through thewindings of relay l z in opposite directions so as to produce; adifierential efiect in the relay. With equal turnsin-thewindingsbf*relay 12, and with equal currents in the plate circuitsof tubes 11 and 16 flowing therethrough, this differential effectresults in a condition of no resultant flux produced in the relay coreand the relay armature remains in a neutral position.

Battery 17 and potentiometer 18 serve to maintain equal normal negativebiases on the grids of tubes 11 and 16, respectively. Whereas the tubes11 and 16 have been assumed similar, two commercial tubes are seldomfound which are identical in their characteristics, and the same normalgrid bias applied to two such tubes would not in general result in equalnormal space currents. Potentiometer 18, as shown, provides anadjustable normal grid bias for the balancing tube 16 so that anyunbalance in the direct current components of the normal space currentsof both tubes will be avoided and the relay armature will normally bemaintained in its central position. For purposes of illustration, it hasbeen assumed in Fig. 1 that tube 16 requires less normal grid bias thantube 11 to produce the same space current. In practice, however, thepotentiometer 18 might be arranged to affect the voltage of the grid oftube 11, or the tubes could be exchanged in position to permit theoperation of the potentiometer. Any difference in space currents will beindicated on the meter 13, which is a differential meter, well known inthe art, indicating the inequality ofcurrents flowing therethrough.

- It has been found, however, that when the incoming signals receivedover conductors 19, which diagrammatically represent any system overwhich a signal is received, are impressed on the grid of amplifier 11,the grid potential-plate current characteristic of the amplifier circuitis sufficiently curved to produce undesirable distortion in the signalsin the output circuit. This is substantiallyovercome by inserting in thepath common to both plate circuits the resistance element 14, therebycausing the volt-ampere output characteristic of the'system to approachconsiderably closer to the trace of a straight line. The effect of thisresistance can be understood from the following discussion.

When an incoming signal on system 19 alters the normal grid potential oftube 11 so as to cause a decrease in space current in this tube, thecurrent flowing through the right hand windings of the relay and thetotal current through resistance 14 are decreased. There is, therefore,a decreased voltagedrop in the external plate circuit of tube 11 and thevoltage of battery 15 effective on the plate of tube 11 is increased.This increased plate voltage tends to producean increasedspace currentin tube 11 in' accordance with the plate a coupling effectbetween tube11 and tube 16. In

the case assumed above, the plate voltage of tube 16 is likewiseincreased but not however, by the,

same amount as in tube 11 as its plate voltage is effected only by thechange of voltage'drop across resistance 14 whereas the plate voltage oftube 11 is also affected by the change of voltage drop in the impedanceof the relay winding in the plate circuit of tube 11. The increase ofplate voltage of tube 16 results ina change in its space current in iaccordance with the plate voltage plate current plate potential changein tube 11 is dependent upon a value of grid voltage other than thenormal bias which is maintained on tube 16. However, the net effect ofthe resistance 14 has been found to be a flattening of the amplifyingcharacteristic of tube 11 and an optimum value of this resistance wasfound as indicated below. The

changes in the circuits of Fig. 1, when an incoming signal in system 19causes an increase in space current of tube 11 will not be discussed asthey may be readily understood from the foregoing discussion. Of course,the net change of current through the windings of relay 12 produces aneffective flux which throws the relay to one or the other ofitstransmitting positions depending upon the incoming signal and asubstantially un distorted signal is retransmitted. I

In Fig. 2 is shown a chart wherein several characteristics ofmilliamperes effective to operate the relay 12 plotted against the gridpotentials for different values of resistance used in the element 14.The characteristic assumes a shape of graph A when the resistanceelement 14 has a value of 1000 ohms; graph B when it has a zero value;and graph C when it has a value of 400 ohms. These graphs are theresultof actual tests and therefore it is readily seen that with aresistance of 400 ohms used for element 14, the distortion is completelyeliminated from the output signals particularly over the operating rangeof the input 15 voltages used in telegraph communication.

What is claimed is: p

1. In a signal impulse repeating system, a source of incoming signalcurrent impulses, a cir-; cuit arrangement havingan input and outputcircuit, said input circuit being responsive to said impulses, a'secondcircuit arrangement having an input and an output circuit, saidinputcircuit being operative independently of said impulses for correctingdistortion of impulsesinthe output circuit of the first mentionedcircuit arrangement, the output circuits of said circuit arrangementshaving a common path, aplurality of relaying devices in said circuitarrangements, an output voltage source in said path and common to saiddevices, an impulse receiving device for repeating impulses receivedfrom said relaying devices, characterized'in this that an impedanceelement is arrangedin said common path for causing said repeating deviceto, operate to either of two'positions every time a signal currentimpulse is impressed on the'input circuit of the first mentioned circuitarrangement whereby the incoming signal" current 'impulses'are repeatedin .the form in which they wereoriginally transmitted.

,2 ..In va signal impulse repeating system, a source of incoming signalcurrent impulses of positive and negative polarity, a circuitarrangement havingran" input and an output circuit, said input circuitbeing responsive to said impulses, a second circuit arrangement havingan input and an output circuit, said input'circuit being operativeindependently of said impulses for correcting distortion of impulsesinthe outputcircuit of the fi'rst mentioned-circuit arrangement,

the output circuits of said circuit arrangements having a common path, aplurality of space discharge devices interconnecting said input circuitsand said output circuits respectively, a voltage source common to saidspace discharge devices and included in said path, an impulse repeatingdevice for repeating impulses received from the output circuits of bothof said discharge devices,

windings on said repeating device arranged to permit the current in theoutput circuit of the first mentioned circuit arrangement to controlsaid repeating device in response to an incoming signal impulse ofpositive polarity and to permit the current in the output circuit of thesecond mentioned circuit arrangement to control said repeating device inresponse to an incoming signal impulse of negative polarity, and animpedance element in said common path whereby the currents that arepermitted to how through said windings are so regulated that theincoming signal current impulses are repeated in the form in which theywere originally transmitted.

3. A signal impulse repeating system according to claim 2, wherein saidimpedance element is a non-inductive resistance arranged in said commonpath to increase and decrease the voltage drop across said secondmentioned circuit arrangement in response to incoming signal impulses ofpositive and negative polarities, respectively.

4. In an impulse repeating system, an amplifier circuit comprising aplurality of space discharge devices, each device comprising a filament,

grid and plate element, said devices having common sources of heatingenergy for said filaments, of normal grid potential and of normal platepotential, repeating means interconnecting the plate circuits of saiddevices wherein a balance of effects of the normal output currents ofsaid devices is maintained, a source of signal impulses of positive andnegative polarity, means for impressing said signal impulses on only oneof said devices to operate said repeating means, and a resistanceelement connected in a common path in the plate circuits of said spacedischarge devices for, respectively increasing and decreasing thepotential drop across the plate circuit of the other of said spacedischarge devices in response to impulses of positive and negativepolarity.

5. In a signal impulse repeating system, an amplifier circuit comprisinga plurality of space discharge devices, each device comprising anode,cathode and impedance control elements, said devices connected withtheir output circuits in pushpull relationship and having common sourcesof anode potential, cathode heating energy and normal impedance controlelement potential, means to produce equal normal space currents in saiddevices, additional means to impress signal impulses on the impedancecontrol element of one of said devices, a common connection path in theanode-cathode circuits of said devices, and an impedance arranged insaid common connection path whereby the system produces undistortedamplified signal impulses.

WILLIAM A. KNOOP.

